Inverse-Designed Diamond Photonics

• Diamond’s material properties and color centers support applications in quantum optics, optomechanics, nonlinear optics, and quantum technologies.Source: Inverse-Designed Diamond Photonics · p. 1
• Limited high-quality diamond thin films and nonrepeatable thinning constrain photonic crystal cavity fabrication.Source: Inverse-Designed Diamond Photonics · p. 1
• Angled etching of bulk diamond produces triangular cross-sections with strongly constrained geometries that limit device design and functionality.Source: Inverse-Designed Diamond Photonics · p. 1
• Quasi-isotropic etching enables rectangular diamond membranes with variable dimensions but introduces feature-size constraints linked to etch depth and undercut.Source: Inverse-Designed Diamond Photonics · p. 1
• Inverse design searches the full parameter space of fabricable devices to address diamond fabrication and design challenges.Source: Inverse-Designed Diamond Photonics · p. 1
• The work demonstrates a compact vertical coupler and integrates inverse-designed couplers and waveguide-splitters with diamond photonic circuits.Source: Inverse-Designed Diamond Photonics · p. 1

• Existing diamond platforms cannot readily use common grating-coupler strategies, while hybrid GaP-on-diamond structures confine the optical field outside diamond emitters.Source: Inverse design of diamond nanophotonic… · pp. 1–2
• Notches provide a practical baseline but have approximately 1% scattering efficiency.Source: Inverse design of diamond nanophotonic… · p. 2
• The inverse-designed vertical coupler occupies 1.0 × 1.0 μm2 and couples directly to a 400 nm waveguide without a taper.Source: Inverse design of diamond nanophotonic… · p. 2
• ≈25% simulated peak efficiency for the coupler versus ≈1% for the notch demonstrates the inverse-designed interface’s performance advantage.Source: Inverse design of diamond nanophotonic… · p. 2
• The coupler targets selective coupling between the free-space TEM00 mode and the waveguide’s fundamental TE mode.Source: Inverse design of diamond nanophotonic… · p. 2
• Theoretical coupling efficiency is limited to 50% because z-axis symmetry sends light equally in the +z and −z directions.Source: Inverse design of diamond nanophotonic… · p. 2
• The optimization maximizes incident-light coupling into the fundamental TE mode while transitioning from continuous permittivity to discrete, fabricable structures.Source: Inverse design of diamond nanophotonic… · p. 2
• Fabrication penalties reduce the 737 nm coupling-efficiency peak from ≈27.5% to ≈25% while enforcing fabricable features.Source: Inverse design of diamond nanophotonic… · p. 3

Polarization and efficiency

• Rotating the input polarization produced a five-fold reduction in transmitted power, consistent with simulations and coupling to a linearly polarized TEM00 mode.The measurement provides experimental evidence for the intended polarization-selective free-space interface.Source: Characterization of diamond vertical… · p. 3

Measurement approach

• The characterization used top-down and side-view measurements, optical microscopy, polarization sweeps, and fiber-collected transmission spectra.The device geometry and measurement workflow are illustrated in the cited figure passages.Source: Characterization of diamond vertical… · pp. 3–4, Figure 3 · p. 4

Polarization and efficiency

• Peak coupling efficiencies reached ≈21 % with a polarization-maintaining fiber and ≈26.5 % with a multimode fiber.The measured bandwidth exceeded 70 nm for the polarization-maintaining fiber and 90 nm for the multimode fiber.Source: Characterization of diamond vertical… · p. 3
• The measured efficiencies agreed well with numerical simulations, while the small fiber-dependent discrepancy indicated efficient coupling into the fundamental free-space mode TEM00.The comparison used both polarization-maintaining and multimode fiber collection.Source: Characterization of diamond vertical… · p. 3

Robustness

• Fifteen devices measured under coarse alignment showed robust transmission, and 30 fabricated devices averaged 24.2 % efficiency across varied doses.The optimization imposed minimum feature sizes to support fabrication yield and robustness.Source: Characterization of diamond vertical… · p. 4

Interface design

• The vertical coupler is compact, robust, and designed for simultaneous fabrication with high-Q/V resonators without additional fabrication steps.It is intended as a free-space interface for cavity quantum electrodynamics and avoids steps that could affect resonator performance.Source: Diamond quantum optical interfaces · p. 4

Cavity characterization

• Nanobeam photonic crystal cavities were measured by coupling a free-space laser into their TE fundamental mode and then into the cavity modes.The comparison included devices using inverse-designed vertical couplers and notches as free-space interfaces.Source: Diamond quantum optical interfaces · pp. 4–5

Performance comparison

• > 550-fold higher cavity-resonance counts were measured with the vertical coupler than with the notch device at comparable Q ≈4000.The measured enhancement matched the 625-fold enhancement expected from simulations.Source: Diamond quantum optical interfaces · p. 5
• The cavity-to-waveguide extraction efficiency was ≈24 times greater for a vertical coupler than for a notch.This result was obtained by directly coupling to the cavity and optimizing collection from each interface.Source: Diamond quantum optical interfaces · p. 5

Experimental consequence

• The higher coupling efficiency reduced some photon-integration times from weeks to minutes.The passage presents this reduction as an experimental consequence of the improved interface performance.Source: Diamond quantum optical interfaces · p. 5

Circuit requirements

• Scaling quantum photonics requires excitation of multiple cavities, interference on beamsplitters, and efficient photon collection and detection.The passage identifies waveguide-splitters as difficult to realize in suspended diamond using angled-etch fabrication.Source: Inverse-designed diamond photonic… · p. 5

Integrated circuit

• The fabricated conceptual circuit combines vertical couplers, waveguide-splitters, and nanobeam photonic crystal cavities with different geometries.The two cavity outputs are interfered and routed through the circuit for off-chip coupling.Source: Inverse-designed diamond photonic… · p. 5
• The inverse-designed waveguide-splitter was designed for a 50:50 splitting ratio with simulated efficiencies of 95 %.The two nanobeam resonances were detuned by < 1 nm because of fabrication imperfections and tuned using gas condensation.Source: Inverse-designed diamond photonic… · p. 5

Implications

• The circuit demonstrates inverse design overcoming limitations of classical photonics for large-scale on-chip quantum optics experiments.The supported extension directions include greater compactness, arbitrary emitter locations, phase matching, and bandwidth-specific optimization.Source: Inverse-designed diamond photonic… · p. 6

Efficiency targets

• Efficiencies above 90 % are achievable with fiber tapers, but their substantially larger footprints motivate compact alternatives.The study compares this trade-off with inverse-designed free-space-waveguide interfaces.Source: Highly efficient free-space-waveguide… · p. 6

Reflective enhancement

• Suspended-air devices reach 67.9 % efficiency, while aluminum back-reflectors raise efficiencies to 72.4 % on SiO2 and 86.6 % in suspended structures.The back-reflector distances are optimized relative to the coupler to match reflected and directly coupled photon phases.Source: Highly efficient free-space-waveguide… · pp. 6–7

Application scope

• These designs are presented as compact alternatives to fiber tapers for single-photon quantum photonic applications.The same interfaces may support optical driving of individual emitters for Raman-based compensation of spectral broadening.Source: Highly efficient free-space-waveguide… · p. 7

• Optimization-based inverse design addresses constraints from cutting-edge diamond nanofabrication.Source: Discussion · p. 7
• Optical free-space couplers and a small diamond photonic circuit achieve high efficiency, compactness, and robustness.Source: Discussion · p. 7
• These compact building blocks enable more complex quantum circuits with varied device components.Source: Discussion · p. 7